This invention relates to an optical matrix switch with a plurality of input and output ports that performs a matrix switching operation on optical input signals supplied to the input ports and generates optical output signals at the output ports.
With the introduction of optical fiber into carrier networks, there has recently been much study of the use of guided-wave optical switches in switching systems.
FIG. 1 is an oblique view showing the structure of an optical matrix switch using this type of guided-wave switch. Sixteen 2.times.2 optical switches 91 are arranged in a 4.times.4 square matrix array on a LiNbO.sub.3 substrate 9 having electro-optic properties. Each of the four input ports 1a, 1b, 1o, and 1d feeds into a row of 2.times.2 optical switches 91, one set of the inputs and outputs of which are connected in series via waveguides 93. The outputs of the last column of 2.times.2 optical switches gI are connected to the four output ports 2a, 2b, 2c, and 2d. The output of each 2.times.2 optical switch 91 that is not connected in series in its row is linked via a waveguide 93 to the input of a 2.times.2 optical switch 91 in the next column that is not connected in series in its row, creating diagonal interconnections among the 2.times.2 optical switches 91. In the prior art thus configured, information entering one of the input ports 1a, 2a, 3a, or 4a is switched by the 2.times.2 optical switches at the crosspoints of the matrix, each of which can be in either the cross or bar state. The number of optical switches in this configuration is the same as in the most elementary optical matrix switch, in which the optical switches are arranged in a square matrix and are connected orthogonally, but this configuration connects any given pair of input and output ports through a constant number of optical switch means and requires only half as many means as the most elementary optical matrix switch in the worst case, so it has the advantages of low loss and reduced crosstalk.
As indicated by the solid and dotted arrows in Fig. 2, however, this prior art configuration is not entirely free from crosstalk. Crosstalk occurs at the third 2.times.2 optical switch 91 in the fourth row when information entering at input port 1b and following the solid arrows meets information entering at input port 1d and following the dotted arrows.
FIG. 2 shows an oblique view of an improved type of optical matrix switch in which crosstalk does not occur. The improved switch comprises sixteen 2.times.2 optical switches 94 arranged in a 4.times.4 square matrix array connected to the input ports 1a, 1b, 1c, and 1d, and a similar array of 2.times.2 optical switches 95 connected to
the output ports 2a 2b, 2c, and 2d. The 2.times.2 optical switches 94 and 95 at the crosspoints of the matrix arrays on the input and output sides are interconnected by waveguides 93 in a pattern that shifts one row lower in the output array for each successive column in the input array.
This arrangement enables the information entering the input ports to undergo matrix switching and be output at the output ports with almost no crosstalk at all, because the same 2.times.2 optical switch never receives differing input information.
Although the prior art as described above achieves much lower levels of crosstalk than an optical matrix switch comprising an elementary square array of 2.times.2 optical switches, it requires both a square array of optical switches on the input side matching the number of input ports, and a similar square array of optical switches on the input side. Since it therefore requires twice as many switches as an optical matrix switch with the elementary square array, the device is larger in size, and there is also a slight increases in the number of optical switch states on the route from the input port to the output port. Another problem with this configuration is that it does not provide a way to reduce light loss.
As a solution to these problems, the inventor has proposed an optical matrix switch that has excellent crosstalk characteristics and comprises approximately the same number of optical switches as an optical matrix switch with an elementary square matrix configuration, but that can be made small in size and has only a small number of optical switches on the route from a given input port to a given output port (Japanese Patent Application No. 267700/1986 filed Nov. 12, 1986). FIG. 3A and FIG. 3B show 4.times.4 and 8.times.8 configurations of this optical matrix switch. The configuration in FIG. 3A comprises input waveguides 1a through 1d, output waveguides 5a through 5d, 2.times.2 optical switches used in the 1.times.2 configuration (2a through 2d) and 2.times.1 configuration (4a through 4d), and 2 .times.2 optical switches 3a through 3d. Circles represent optical switches; lines represent waveguides linking the switches. These optical switches and waveguides are fabricated on &he same substrate.
The configuration in FIG. 3B comprises input waveguides 1a through 1h, output waveguides 5a through 5h, 2.times.2 optical switches used in the 1.times.2 configuration 2.sub.1a through 2.sub.1h, 2.times.2 optical switches used in the 2.times.1 configuration 4.sub.2a through 4.sub.2h, and 4.times.4 optical switches 7a through 7d consisting of 2.times.2 optical switches structured as in FIG. 3A. The input Waveguides 1a through 1h and 1.times.2 optical switches 2.sub.1a through 2.sub.1b in FIG. 3B can be divided into two groups: those labeled with subscripts from a through d and those labeled with subscripts from e through h. The output 2.times.1 optical switches 4.sub.2a through 4.sub.2h and the output waveguides 5a through 5h can also be divided into two groups.
The input ports 1a through 1d are selectively connected by the input optical switches 2.sub.1a through 2.sub.1d to one of the 4.times.4 optical switches 7a and 7b connected to the two output port groups. Similarly, the input ports 1e through 1h are selectively connected by the input optical switches 2.sub.1e through 2.sub.1h to one of the 4.times.4 optical switches 7c 7 and 7d connected to the two output port groups. The output 2.times.1 optical switches 42a through 42h select one of the 4.times.4 optical switches 7a and 7b or 7c and 7d connected to the two groups of input ports 1a through 1d and 1e through 1h and connect them to the output ports. In a given group, the output port that is selected depends on the operation of the 4.times.4 optical switches 7a through 7d.
A major feature of this type of optical matrix switch is that it Is smaller than the square matrix type in general use.
A problem in the configuration described above is that light loss occurs at the points of intersection of the waveguides, degrading the crosstalk characteristics. Consider, for example, the light signal 41 traveling through the waveguide 7c-4.sub.2a in FIG. 3C. If the light signal 42 is present in the waveguide 7a-4.sub.2b crosstalk will occur at the point of intersection 43 of the waveguides 41 and 42. Suppose that the amount of crosstalk into the waveguide 7a-4.sub.2a at the point of intersection 43 is 20 dB with respect to the strength of the light signal 42, and that the loss at the points of intersection 44a through 44f and 43 is 2dB. If the light signals 41 and 42 had the same initial strength, immediately after passing the point of intersection 43 the light signal 42, since it has passed through points of intersection 44a through 44f as well as 43, has lost 14dB in strength. The amount of crosstalk therefore increases to 6dB with respect to the light signal 41.